(American Journal of Pathology. 2001;159:1061-1068.)
© 2001 American Society for Investigative Pathology
Elevation of Cystatin C in Susceptible Neurons in Alzheimers Disease
Amy Deng*,
Michael C. Irizarry*,
Roger M. Nitsch
,
John H. Growdon
and
G. William Rebeck*
From the Memory Disorders Clinic
and the Alzheimers Research Unit,*
Department of
Neurology, Massachusetts General Hospital, Boston, Massachusetts; and
the Division of Psychiatric Research,
Universtiy of Zurich, Zurich, Switzerland
 |
Abstract
|
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A common polymorphism in the cystatin C gene is associated with
increased risk of developing Alzheimers disease (AD). To explore
possible neuropathological consequences of this genetic
association, we examined expression of cystatin C in brains
from 22 AD and 11 control patients by immunohistochemistry. In the
temporal cortex of all AD brains, there was strong cystatin C
immunostaining of neurons and activated glia, whereas staining
was absent or minimal in 7 of the 11 control brains. Neuronal staining
of cystatin C in AD brains was primarily limited to pyramidal neurons
in cortical layers III and V, which are the neurons most
susceptible to cell death in AD. The increase in cystatin C staining in
AD was independent of cystatin C genotype. Immunostaining of cystatin C
within neurons showed a punctate distribution, which
co-localized with the endosomal/lysosomal proteinase, cathepsin
B. A primarily glial source for cystatin C was suggested by parallel
studies using in situ hybridization of mouse brain. In
human AD brain, there was little co-localization of cystatin C
with parenchymal Aß deposits, although a small fraction of
cerebral blood vessels and neurofibrillary tangles were cystatin
C-positive. The regional distribution of cystatin C neuronal
immunostaining also duplicated the pattern of neuronal susceptibility
in AD brains: the strongest staining was found in the entorhinal
cortex, in the hippocampus, and in the temporal cortex;
fewer pyramidal neurons were stained in frontal,
parietal, and occipital lobes. These neuropathological
observations reinforce the association between cystatin C and
AD, and support a model of cystatin C involvement in the
process of neuronal death in AD.
 |
Introduction
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Cystatin C is a cysteine protease inhibitor that inhibits the
cathepsin family of lysosomal proteases (cathepsin B, H, L, and
S).1
It is a 14-kd secreted protein (120 amino
acids),2
present in cerebrospinal fluid at high
concentrations.3
A mutant form of cystatin C forms the
cerebrovascular amyloid in hereditary cerebral hemorrhage with
amyloidosis, Icelandic-type, because of a leucine-to-glutamine
substitution at position 68.4
However, cystatin C is only
found in a subset of cerebral amyloid deposits in sporadic cases of
cerebral amyloid angiopathy.5-8
In these cases, amyloid
is primarily composed of the 4-kd Aß peptide, which is also found in
the parenchymal amyloid deposits in the brain of Alzheimers disease
(AD) patients.
In addition to the rare Icelandic
mutation of the cystatin C gene (CST3), there are two
common haplotypes of CST3 (A and B) that differ from each
other at three sites: two single base pair changes in the promoter
region and one in the signal peptide domain that causes an amino acid
substitution (alanine to threonine).9
Recently,
case-control studies found associations of CST3 with
increased risk of late onset AD.10,11
In the larger
series, Finckh and colleagues10
found the BB genotype in
<2% of control individuals but in 5 to 9% of AD patients, and in
even higher percentages in those patients with the latest ages of
onset. These data suggest that inheritance of CST3 BB
increases the risk of AD, particularly with onset after age 75. To
determine how changes in the CST3 gene could affect the
neuropathological processes of AD, we examined the distribution of the
cystatin C protein in AD brain tissue.
 |
Materials and Methods
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Antibodies
For cystatin C immunohistochemistry, two rabbit polyclonal
antibodies were used (Upstate Biotechnology, Lake Placid, NY, and
Biogenesis, Kingston, NH). Western blots of human brain extracts
with cystatin C antibodies revealed a strong band of
14 kd,
corresponding in size to purified cystatin C (Figure 1)
. Cathepsin B was detected with a sheep
polyclonal antibody (ICN Biochemicals, Irvine, CA). Aß was detected
with the mouse monoclonal antibody 10D5 (courtesy of Dr. Peter Seubert,
Elan Pharmaceuticals).12
Phospho-tau was detected
with the mouse monoclonal antibody PHF-1 (courtesy of Dr. Peter Davies,
Albert Einstein University, New York, NY).13
)

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Figure 1. Immunoblot of cerebrospinal fluid and brain for cystatin C. The
specificity of anti-cystatin C antibody was demonstrated using standard
cystatin C (lane
1), human cerebrospinal fluid
(lane 2), and
human brain tissue (lanes 3 and
4). Antibodies recognized monomeric
cystatin C at 14 kd; dimeric cystatin C is seen with the standard
cystatin C. A second band of reactivity is seen at >50 kd in
cerebrospinal fluid and brain, possibly reflecting the presence of
albumin.
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Brain Tissue
Temporal cortex of 22 neuropathologically confirmed AD cases and
11 control brains were collected from the Massachusetts Alzheimer
Disease Research Center brain bank (Dr. E. T. Hedley-Whyte,
director). The AD group consisted of 9 males and 13 females (average
age, 80 years; SD, 7); the control group consisted of five males and
five females (average age, 73 years; SD, 20). In addition, frontal
cortex, occipital cortex, parietal cortex, and cerebellum were obtained
from five AD cases. Postmortem intervals averaged 19 hours. Tissue was
fixed in paraformaldehyde-lysine-metaperiodate for 24 hours at 4°C,
and cryoprotected in 15% glycerol in 0.1 mol/L Tris-buffered saline
(pH 7.4). Fifty-µm sections were prepared using a freezing-sledge
microtome. The control brain tissues were screened for AD
neuropathological changes by immunostaining for amyloid deposits and
neurofibrillary tangles (Table 1)
. Braak
and Braak14
staging was determined by the distribution of
PHF-1-stained neurofibrillary tangles.
DNA was isolated from 50 mg of frozen brain tissue using Qiagen DNA
extraction kits (Qiagen, Valencia, CA). The three linked polymorphisms
in the promoter and signal peptide of CST3 were analyzed by
a polymerase chain reaction-restriction enzyme assay as
described.10
Immunoblotting
Standard cystatin C (50 ng; Calbiochem, La Jolla, CA), human
cerebrospinal fluid (15 µl), and human brain extract (10 µg protein
in 50 mmol/L Tris, pH 8.0, 0.5 mol/L NaCl, 0.1% Triton X-100) were
separated by 4 to 20% polyacrylamide gel electrophoresis. Proteins
were transferred to polyvinylidene difluoride membrane, probed with
anti-cystatin C antibody, and visualized by chemiluminescence.
Immunohistochemistry
Brain tissue was immunostained using anti-cystatin C (1:500 to
1:1000), 10D5 (1:1000), PHF-1 (1:200), or anti-cathepsin B (1:250).
Antibodies were detected using horseradish peroxidase-linked secondary
antibodies (anti-rabbit for cystatin C, anti-mouse for Aß, and
phospho-tau; Jackson Immunoresearch, West Grove, PA) and visualized
with 3,3'-diaminobenzidine. For fluorescent double labeling, tissue was
initially co-incubated with two primary antibodies. Cystatin C
antibodies were detected with Cy3-linked or
(4,4-difluoro-5,7-dimethyl-4-bova-3a,4a-diaza-5-indacene)
(BODIPY)-linked anti-rabbit secondary antibodies; Aß and
phospho-tau antibodies were detected with BODIPY-linked anti-mouse
secondary antibodies; cathepsin B antibodies were detected with
Cy3-linked secondary antibodies. Fluorescence was observed using the
BioRad MRC-1024 confocal microscope (Cy3: excitation at 568 nm,
emission at 605 nm, BODIPY: excitation at 488 nm, emission at 522 nm)
(Bio-Rad, Richmond, CA). Controls without primary antibodies were
included for each experiment.
In Situ Hybridization
Cystatin C in situ hybridization was performed
according to published protocols on 14-µm sagittal sections from five
APP transgenic and three nontransgenic mice (three 12-month-old PDAPP
homozygotes,15
three 2- to 15-month-old nontransgenic
mice, and two 15-month-old Tg2576 heterozygotes16
) using
mouse cystatin C (GenBank accession no. M59470) bases 181 to 226 45-mer
sense and antisense oligonucleotide probes.17,18
Hybridized slides were dipped in Amersham LM-1 emulsion (Amersham,
Arlington Heights, IL), stored at 4°C for 4 weeks, developed in Kodak
D-19 (Eastman-Kodak, Rochester, NY), and counterstained with thionin
and thioflavin S.
 |
Results
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We immunostained the temporal cortex from 11 control and 22 AD
brains for cystatin C. In 7 of the 11 control brains, there were no
cystatin C-positive cells visible (Figure 2A)
, whereas there was strong cystatin C
immunostaining in all AD brains (Figure 2B)
. Cystatin C
immunoreactivity was confined primarily to pyramidal neurons, of
cortical layers III and V, although neurons in layer II were also often
positive. This distribution of neuronal staining demarcated those
neurons most susceptible to loss in AD cortex. To determine whether
other susceptible neurons were also cystatin C-positive in the AD
brain, we examined the entorhinal cortex and the hippocampus. Neurons
in all hippocampal subfields were positive, with the strongest staining
observed in CA1 (Figure 2C)
. Similarly, entorhinal cortex neurons were
also strongly positive for cystatin C (Figure 2D)
. Of the 22 AD brains,
15 were the CST3 AA genotype, 6 were AB, and 1 was BB;
strong neuronal staining was observed for the brains within each
genotype subgroup. Thus, regardless of CST3 genotype,
cystatin C is found in neurons throughout the temporal lobe that are
susceptible to loss in AD.

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Figure 2. Cystatin C is increased in AD brain. Temporal lobes of control
(A) and AD
(BD) were
immunostained for cystatin C. Cortex showed no staining in the control
case (A), but
strong staining of layer III and V pyramidal neurons in AD brain
(B). Cystatin
C was also strongly present in neurons of CAI
(C) and
entorhinal cortex
(D). Original
magnifications: x31 (A and
B), x80
(C and
D).
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Four of the 11 control brains showed a distribution of cystatin C that
was similar to that seen in AD, with prominent staining of pyramidal
neurons in cortical layers III and V, and glial staining throughout the
cortex. There was no correlation of the cystatin C-positive controls
with age. Two of the four cystatin C-positive controls showed strong AB
immunostaining (Table 1)
. In contrast, none of the seven cystatin
C-negative controls showed strong Aß immunostaining.
We examined the cellular distribution of cystatin C within the AD
brain. The cystatin C in pyramidal neurons was present in a vesicular
pattern (Figure 3A)
. Cystatin C was also
present in glia throughout the cortex (Figure 3B)
. The morphology of
the cystatin C-positive glia suggested that they were strongly
activated. Cystatin C-positive glia and neurons were observed together
in some fields, and separately in others.

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Figure 3. Cystatin C is present in both neurons and glia of AD brain. Temporal
cortex of AD brain immunostained for cystatin C revealed a vesicular
pattern in pyramidal neurons
(A) as well as
staining in activated glia in the white matter
(B). Original
magnifications, x500.
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The vesicular pattern of cystatin C immunoreactivity in neurons
suggested its presence in endosomes and lysosomes. We immunostained AD
brain tissue for cathepsin B, an endosomal/lysosomal
protein19
and a protease that is inhibited by cystatin
C.1
Cathepsin B and cystatin C showed similar punctate
patterns of staining in neurons, with almost complete co-localization
in intraneuronal vesicles (Figure 4
, in
yellow). These data demonstrate the presence of cystatin C in endosomes
and lysosomes in AD brain.

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Figure 4. Co-localization of cystatin C with cathepsin B. Temporal cortex of AD
brain was double-immunostained for cathepsin B
(in red) and cystatin C
(in green), demonstrating
vesicular co-localization of this protease-inhibitor pair
(in yellow). Original
magnifications, x1000.
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This vesicular cystatin C could be produced within neurons, or be
produced in other cells and enter neurons through an endocytic pathway.
To gain insight into the cellular source of cystatin C in the brain, we
performed in situ hybridization of mouse brain and mouse
models of Aß deposition. Emulsion-dipped sections demonstrated
predominantly glial expression of cystatin C, with emulsion granules
overlying glia in white matter and interspersed between neurons in the
cortex and subcortical regions (Figure 5A)
. In APP transgenic mice, glia
surrounding thioflavin S-labeled amyloid deposits also expressed
cystatin C (Figure 5B)
; sense probes yielded no detectable signal.
Microglia could not be distinguished from astrocytes, and neuronal
hybridization was minimal. Thus, cystatin C seems primarily produced by
glial cells in the brain, which are activated in the vicinity of
amyloid deposits.

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Figure 5. In situ hybridization of cystatin C in mouse brain.
Emulsion-dipped cystatin c in situ hybridization in a
15-month-old nontransgenic mouse
(A)
demonstrates accumulations of silver granules over glia in the dentate
gyrus of the hippocampus (nissl counterstain in
gray). Glia surrounding an amyloid deposit in a
12-month-old homozygous PDAPP mouse
(B) also
express cystatin C (nissl
counterstain). Scale bars, 50 µm.
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We were struck by the absence of cystatin C immunostaining of amyloid
deposits in the sections of AD brain examined. Figure 6
shows the parahippocampal gyrus from
one AD case, stained for cystatin C (Figure 6A)
, Aß (Figure 6B)
, or
phosphorylated tau (Figure 6C)
. Despite the presence of large amounts
of cortical Aß, no cystatin C was observed in these parenchymal
deposits (Figure 6, A and B)
. A second polyclonal cystatin C antibody
was used on sections from AD brains; it also identified cortical
neurons without obvious staining of Aß deposits. The layers of
cystatin C-positive neurons did partially overlap with phosphorylated
tau-positive neurons (Figure 6, A and C)
, raising the possibility that
cystatin C and neurofibrillary tangles did occasionally co-localize.

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Figure 6. Neuronal cystatin C in regions of AD neuropathological changes.
Parahippocampal gyrus of AD patient immunostained for cystatin C
(A), Aß
(B), and
phospho-tau
(C). The
pattern of cystatin C staining is primarily independent of plaques and
tangles. Original magnifications, x80.
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The pattern of co-localization of cystatin C with Aß and
neurofibrillary tangles was more closely examined by confocal
microscopy using double immunofluorescence (Figure 7)
. Cystatin C (in red) immunostained
neurons in the vicinity of parenchymal Aß deposits (in green) without
evidence of co-localization with the plaques (Figure 7A)
. In contrast
to the plaques, there were some cerebrovascular deposits that
immunostained strongly with both Aß and cystatin C (Figure 7A
, inset,
in yellow). There was partial co-localization of cystatin C with
phospho-tau in some cortical neurons (Figure 7B
, arrows), but most
neurofibrillary tangles were cystatin C-negative and most cystatin
C-positive neurons were phospho-tau-negative. Control
immunofluorescence studies without primary antibodies revealed no
specific extracellular or intracellular staining.

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Figure 7. Confocal imaging of cystatin C with Aß and phospho-tau immunostains.
A: Temporal lobe of AD brain was co-labeled with antibodies
against cystatin C (in
red) and Aß (in
green). Cystatin C and parenchymal Aß did not
co-localize, although cystatin C and cerebrovascular amyloid did
occasionally co-localize (see inset,
in yellow). B: Temporal lobe of AD
brain was co-labeled with antibodies against cystatin C
(in red) and phospho-tau
(in green). Cystatin C
and neurofibrillary tangles mostly labeled separate structures,
although there were regions of co-localization
(in yellow, denoted by
arrows). Original magnifications,
x400.
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We examined the distribution of cystatin C staining in other cortical
regions of five AD brains (Figure 8)
.
Cystatin C-positive pyramidal neurons were found in temporal (Figure 8A)
, frontal (Figure 8B)
, parietal (Figure 8C)
,and occipital lobes
(Figure 8D)
. This staining respected a laminar distribution in all
brain regions, and was consistently strongest in the temporal lobe. We
observed diffuse laminar immunostaining in the cerebellar cortex and
cerebellar dentate nucleus, with cellular staining also apparent in the
dentate nucleus. Cystatin-C-positive glia were found in regions of
cortex and white matter in all regions. The strongest neuronal staining
in the temporal cortex compared to other brain regions again mirrored
the differential susceptibility of brain regions in AD.

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Figure 8. Regional distribution of cystatin C in AD brain. Various regions of an
AD brain were immunostained for cystatin C: temporal lobe
(A), frontal
lobe (B),
parietal lobe
(C), occipital
lobe (D).
Neuronal staining was more pervasive in temporal lobe than in other
brain regions. Original magnifications, x80.
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Discussion
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The genetic associations of CST3 with
AD10,11
led us to examine the neuropathological
distribution of the cystatin C protein in AD brain. We found a dramatic
neuronal staining in the 22 AD brains we examined, but no or minimal
staining in 7 of the 11 aged control brains examined. In addition, we
found activated glia that were cystatin C-positive in the AD brains,
thereby confirming a previous report regarding glial staining in
AD.20
We and others have found cystatin C message
primarily in glia (Figure 5)
21
) as well as choroid
plexus.22
We suggest that the immunostaining of glia
represents cystatin C that is being secreted, and the immunostaining of
neurons represents cystatin C that is endocytosed. We observed this
pattern of immunostaining with two cystatin C antibodies, and found it
present in AD brains regardless of CST3 genotype, suggesting
that cystatin C involvement in AD pathogenesis is not limited to those
individuals with the at-risk CST3 genotypes.
Our data extend a recent report of neuronal cystatin C immunostaining
in cortical neurons23
by demonstrating the topographical
pattern of neuronal staining. Neuronal staining was most intense in the
temporal lobe compared with the frontal, parietal, and occipital lobes
and least intense in the cerebellum (Figure 8)
. The strongest
immunostaining was found in pyramidal neurons, and more specifically,
in neurons of cortical layers III and V (Figure 2)
. Neurons of the
entorhinal cortex and hippocampal subfields were very strongly stained
(Figure 3)
. This pattern of cystatin C immunostaining recapitulates the
hierarchical vulnerability of neurons in the AD
brain.14,24
We hypothesize that cystatin C
immunoreactivity identified neurons that were damaged by AD
neuropathological processes. This hypothesis is further supported by
our observation that cystatin C and neurofibrillary tangles
occasionally co-localize within neurons (Figure 7B)
.
In both this study and that of Yasuhara and colleagues,20
there were a minority of aged control brains that were strongly
cystatin C-positive. In our study, two of the cystatin C-positive
control brains showed substantial amounts of Aß deposits whereas none
of the seven cystatin C-negative brains showed as much Aß (Table 1)
.
These data imply there may be some connection between the accumulation
of Aß early in AD and cystatin C accumulation in intraneuronal
vesicles. This situation parallels precisely the changes in early
stages of AD observed for lysosomal proteases, the class of proteases
inhibited by cystatin C. Lysosomal proteases (including cathepsins B
and D) are up-regulated in the AD brain19,25
and these
changes occur as the earliest markers of AD neuropathological
changes.26
We found cystatin C and cathepsin B
co-localized in these neuronal vesicles (Figure 4)
, which are increased
in size and number in the AD brain.19,27
These neurons
have also been reported to possess vesicular accumulations of
Aß42.28
We hypothesize that cystatin C is accumulating
in endocytic vesicles to aid in inhibition of the increased levels of
lysosomal proteases (eg, cathepsin B), which may be a neuronal response
to intracellular Aß.
We did not observe frequent staining of parenchymal amyloid deposits in
AD brains (Figure 6)
, although we did note many instances of
cerebrovascular amyloid that were cystatin C-positive (Figure 7A)
. In
studies of AD, cystatin C has been observed in some parenchymal amyloid
deposits in AD,23
but more often in amyloid-surrounding
blood vessels.5,6
Cystatin C is also found in
cerebrovascular amyloid in cases of sporadic cerebral amyloid
angiopathy5-8
in addition to the cases of cerebral
amyloid angiopathy caused by the cystatin C L68Q
mutation.4
The weak staining of parenchymal amyloid
deposits with cystatin C contrasts with the prominent co-localization
of cystatin C and Aß in plaques observed in transgenic APP
mice.29
These apparent disparities between parenchymal and
cerebrovascular amyloid and between human and mouse brain tissues
remain unexplained, but it may provide clues into the process of Aß
deposition.
One basic question is whether elevated cystatin C is specific to the
chronic neurodegeneration of AD, or whether it is also present in
conditions of acute neuronal injury. In models of transient global
ischemia in rodents, cystatin C was found elevated in hippocampal
neurons,30
and well as glia.31
Similarly,
after facial nerve axotomy, microglial cystatin C mRNA was up-regulated
in the damaged facial nucleus.21
Cystatin C has recently
been found to act as an autocrine or paracrine factor independent of
its function as a protease inhibitor; its presence in the adult brain
may be related to its role in promoting neurogenesis in neural stem
cells.32
Together these findings suggest that cystatin C
may be a marker for damaged or degenerating neurons in conditions other
than just AD.
The pattern of cystatin C immunostaining in susceptible neurons and its
elevation in models of neuronal loss have focused our attention to a
role for cystatin C in neuronal protection in AD. Our working model is
that cystatin C is endocytosed by damaged neurons and targeted to the
lysosome. These neurons would have begun to accumulate lysosomal
proteases and hydrolases, and cystatin C could act as an inhibitor of
some of these proteases to protect the cell from excessive lysosomal
dysfunction. If the neuronal insult is too profound or too persistent,
the neurons may accumulate neurofibrillary changes and degenerate.
 |
Acknowledgements
|
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We thank the Massachusetts Alzheimer Disease Research Center brain
bank (Dr. E. T. Hedley-Whyte, director) for access to brain
tissue; Brad Hyman for helpful discussions; Efrat Levy for sharing her
unpublished data; Dr. Peter Seubert for the Aß antibody; and Dr.
Peter Davies for the phopsho-tau antibody.
 |
Footnotes
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Address reprint requests to G. William Rebeck, Alzheimer Research Unit, 114 16th St., Charlestown, MA 02129. E-mail:
rebeck{at}helix.mgh.harvard.edu
Supported by the Alzheimers Association (grant IIRG-99-1755), the National Institutes of Health (grant AG 05134), and the Luttrell Foundation for Alzheimers Research.
Accepted for publication June 11, 2001.
 |
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